Sheet-form optical waveguides are suggested for optical communication/optical circuits, such as an optical cable and an optical waveguide, and for displays, such as a liquid crystal display, a plasma display, an EL display, and a projection screen.
For example, for optical communication/optical circuits, Patent Document 1 discloses a signal transmitting bus wherein light transmitting layers, which receives signal light and then diffuses and transmits the received signal light, and light blocking layers, which blocks the mixing of the signal light between adjacent ones out of the light transmitting layers, are alternately laminated. Such an optical waveguide has advantages that: the amount of crosstalks or electromagnetic noises therefrom is small; the waveguide is easily positioned; and a circuit board can be freely put on and taken off from the waveguide. However, this optical waveguide is of a step index type (SI type), and thus the waveguide is unsuitable for high-speed transmission. Additionally, there remains a problem that the loss of transmission is large at the time of bending the waveguide into a large curvature and using the bent waveguide.
As a manner capable of solving such problems of the SI type, for example, Patent Document 2 discloses an optical waveguide of a graded-index type (GI type), which has a refractive index distribution along its depth direction from its sheet surface. However, the GI type optical waveguide is formed by infiltrating and dispersing a sublimable or volatile organic compound, as disclosed in, for example, Patent Document 2; therefore, it is difficult to control the distribution precisely into a refractive index distribution like a square distribution ideal for the GI type optical waveguide, wherein mode distribution is hardly generated. In a high-temperature and high-humidity environment, the infiltrated and dispersed compound frequently diffuses, thereby causing a problem that the refractive index distribution changes with time.
For displays such as a screen, Patent Document 3 discloses a technique of controlling the refractive index distribution of an optical waveguide in accordance with the concentration distribution of nano particles having a large aspect ratio. However, the concentration distribution of the nano particles is not mechanically controlled. Thus, it is essentially difficult to attain a refractive index distribution high in precision, and there is also a problem that loss based on scattering is large.
Meanwhile, various films wherein a thermoplastic resin is laminated into a multilayered form are suggested. There are disclosed, for example, a product wherein a multi-layered film excellent in tearing resistance is caused to adhere onto a surface of a piece of glass, thereby preventing the glass from being damaged or scattered to a large degree (see, for example, Patent Documents 4 to 6); and a film wherein resins having different refractive indexes are alternately laminated into a multilayered form, thereby reflecting specific wavelengths selectively (see, for example, Patent Documents 7 to 9). Of these, the film on which specific wavelengths are selectively reflected acts as a filter for transmitting specified light rays or reflecting the specific light, and is used as a film for backlight in a liquid crystal display or the like.
However, the conventional laminated films give a large loss resulting from reflection based on the refractive index difference between layers therein, and cannot be applied to optical waveguides.
[Patent Document 1] Japanese Patent Application Laid-Open (JP-A-) No. 9-270752 (page 2)
[Patent Document 2] JP-A No. 2003-322742 (page 2)
[Patent Document 3] JP-A No. 2004-133473 (page 2)
[Patent Document 4] JP-A No. 6-190995 (page 2)
[Patent Document 5] JP-A No. 6-190997 (page 2)
[Patent Document 6] JP-A No. 10-76620 (page 2)
[Patent Document 7] JP-A No. 3-41401 (page 2)
[Patent Document 8] JP-A No. 4-295804 (page 2)
[Patent Document 9] Japanese Patent Application National Publication No. 9-506837 (page 2)